Rabbit with megacolon?

My rabbit is a 3 year old mini rex, turning 4 in August. 2 or 3 weeks ago we noticed him not eating or drinking much at all, and he was just lying in one place almost all day. That night we also noticed he had noticeably small poops in little piles. He started eating and drinking a lot more the next day and we took him to the vet the day after that. They said he had mild GI stasis and told us how to treat it. The following days he seemed completely normal, eating and drinking a lot, but no more than usual. Yesterday he started to seem a bit less energetic, but nothing to cause alarm. Last night, I noticed his urine was also a very strange color. Instead of his usual yellowish-clearish pee, this was very milky looking and a very pale yellow. Almost like vanilla pudding. His feces were also a little smaller than usual, but nothing like they were a few weeks ago with stasis. He seems to still be going as much as normal though. Today I went out to see him some more since he seemed to be lying in one place a lot again, and I noticed he had produced a giant poop. Like 4 or 5 times the size of his usual poops. It was shaped like an egg, a little less wide that a quarter, and about as long as a quarter and a half or so. He was still making smaller-sized poops, too, the big one was surrounded by the smaller ones. After I sat down he went running over to me and started to hog down some pellets. I immediately replaced those with some romaine lettuce, dandelions, and timothy hay (he had already had some, I just got him fresh). He ate a LOT, like 10 minutes of non-stop eating (keep in mind a lot of that time was spent chewing, so it wasn't like he was constantly swallowing for 10 minutes). This was about 30-45 minutes ago, I just looked out and he's sitting upright by his food, not hunched or lying down. Sitting upright while sleeping isn't uncommon for him, he does it more than lying or hunched. So he's seeming pretty normal. It seem like he drank a bit, too. Does it sound like my bunny's in danger? I read that giving him fresh, moist greens like the lettuce I gave him is good. Is there anymore I can do? Thank you

The large poop could simply have been due to his gut slowdown, once he passed it, it sounds like he was feeling better and wanting to eat again, so it doesn't really sound like megacolon to me. I don't think he's in danger, he's eating well, and as long as he's eating plenty of hay and continuing to poop, it doesn't sound like there's an issue.